This invention relates to an improved amorphous alloy device having improved photoresponsive characteristics. The invention has its most important application in making multiple cell photoresponsive alloy devices having tailor made band gaps for specific photoresponsive applications including photovoltaic devices such as solar cells of a p-i-n, Schottky or MIS (metal-insulator-semiconductor) type.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 per cent) crystalline solar cells. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extermely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorpiton in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the addition of grain boundaries and other problem defects.
An additional shortcoming of the crystalline material, for solar applications, is that the crystalline silicon band gap of about 1.1 eV inherently is below the optimum band gap of about 1.5 eV. The admixture of germanium, while possible, further narrows the band gap which further decreases the solar conversion efficiency.
In summary, crystal silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices manufactured with amorphous silicon can eliminate these crystal silicon disadvantages. Amorphous silicon has an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon can be made faster, easier and in larger areas than can crystal silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. Le Comber or Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196 (1975), toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas of silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material a gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited including supposedly substitutional phosphorus or boron dopant and was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the amorphous material approximate more nearly the corresponding crystalline material.
D. I. Jones, W. E. Spear, P. G. LeComber, S. Li, and R. Martins also worked on preparing a Ge:H from GeH.sub.4 using similar deposition techniques. The material obtained gave evidence of a high density of localized states in the energy gap thereof. Although the material could be doped the efficiency was substantially reduced from that obtainable with a-Si:H. In this work reported in Philsophical Magazine B, Vol. 39, p. 147 (1979) the authors conclude that because of the large density of gap states the material obtained is ". . . a less attractive material than a-Si for doping experiments and possible applications."
In working with a similar method of glow discharge fabricated amorphous silicon solar cells utilizing silane, D. E. Carlson attempted to utilize germanium in the cells to narrow the optical gap toward the optimum solar cell value of about 1.5 eV from his best fabricated solar cell material which has a band gap of 1.65-1.70 eV. (D. E. Carlson, Journal of Non Crystalline Solids, Vol. 35 and 36 (1980) pp. 707-717, given at 8th International Conference on Amorphous and Liquid Semi-Conductors, Cambridge, Mass., Aug. 27-31, 1979). However, Carlson has further reported that the addition of germanium from germane gas was unsuccessful because it causes significant reductions in all of the photovoltaic parameters of the solar cells. Carlson indicated that the degradation of photovoltaic properites indicates that defects in the energy gap are being created in the deposited films. (D. E. Carlson, Tech. Dig. 1977 IEDM, Washington, D.C., p. 214).
The concept of utilizing multiple cells to enhance photvoltaic device efficiency was discussed at least as early a 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures discussed utilized p-n junction crystalline semiconductor devices, but the enhancement concepts are similar whether amorphous or crystalline devices are utilized. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase Voc. The stacked cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer.
Many articles on crystalline stacked cells following Jackson, have been reported and more recently several articles dealing with a Si-H materials utilized in stacked cell have been published. Marfaing proposed utilizing silane deposited amorphous Si-Ge alloys in stacked cells, but did not report the feasibility of doing so. (Y. Marfaing, Proc. 2nd European Communities Photovoltaic Solar Energy Conf., Berlin, West Germany, p. 287, (1979).
Hamakawa et al, reported the feasibility of utilizing a-SiH in a configuration which will herein be defined as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer between each of the cells. Each of the cells was made of a-Si:H material of the same band gap in a p-i-n junction configuration. Matching of the short circuit current (Jsc) was attempted by increasing the thickness of the cells in the serial light path. As expected the overall device Voc increased and was proportional to the number of cells.
In a recent report on increasing the cell efficiency of multiple-junction (stacked) solar cells of amorphous silicon (a-Si:H) deposited from silane in the above manner, it was reported that "[g]ermanium has been found to be a deleterious impurity in a-Si:H, lowering its J.sub.sc exponentially with increasing Ge . . . " From their work as well as that of Carlson, Marfaing and Hamakawa they concluded that alloys of amorphous silicon, germanium and hydrogen "have shown poor photovoltaic properties" and thus new "photovoltaic film cell materials must be found having spectral response at about 1 micron for efficient stacked cell combinations with a-Si:H." (J. J. Hanak, B. Faughnan, V. Korsun, and J. P. Pellicane, presented at the 14th IEEE Photovoltaic Specialists Conference, San Diego, California, Jan. 7-10, 1980).
The incorporation of hydrogen in the above method not only has limitations based upon the fixed ratio of hydrogen to silicon in silane, but, also, various Si:H bonding configurations introduce new antibonding states which can have deleterious consequences in these materials. Perhaps most importantly hydrogen is incapable of compensating or removing defect states produced by the introduction of germanium into the alloy. Therefore, there are basic limitations in reducing the density of localized states in these materials which are particularly harmful in terms of effective p as well as n doping. The resulting density of states of the silane deposited materials leads to a narrow depletion width, which in turn limits the efficiencies of solar cells and other devices whose operation depends on the drift of free carriers. The method of making these materials by the use of only silicon and hydrogen also results in a high density of surface states which affects all the above parameters. Further, the previous attempts to decrease the band gap of the material while successful in reducing the gap have at the same time added states in the gap. The increase in the states in the band gap results in a decrease or total loss in photoconductivity and is thus counterproductive in producing photoresponsive devices.
Grading of the band gap of crystalline p-n junction silicon photovoltaic solar cells was suggested in 1960 by Wolf, "Limitations And Possibilities For Improvement Of Photovoltaic Solar Energy Converters," Proc. IRE, VOL. 48, pp. 1246-1263; 1960. At the time of his report, Wolf concluded that the preparation of crystalline semiconductor materials with graded energy gaps was technically possible but their application to photovoltaic cells did not appear to be very useful because of the loss in V.sub.oc.
D. E. Carlson, in the Journal of Non-Crystalline Solids, Vol. 35 and 36 (1980), pp. 707-717 also suggested that grading the band gap in silane deposited devices "may improve the device performance". No suggestion was made however on how to grade the band gap over any significant range, other than the suggestion of adding germanium which Carlson also has reported as a failure. No suggestion of increasing the band gap was made whatsoever.
Later, Carlson suggested that the band gap of amorphous silicon solar cells in a silane environment may be graded to provide a potential gradient that would assist in the collection of photogenerated carriers. However, it was not known how such gradient could be accomplished without adding germanium to the amorphous Si:H which, as previously noted, was recognized by Carlson to cause significant reduction in all of the photovoltaic parameters of the solar cells.
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in the atmosphere of mixture of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as a compensating agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states in the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process described above. The above described p and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process. Neither process produced efficient p-doped materials with sufficiently high acceptor concentrations for producing commercial p-n or p-i-n junction devices. The n-doping efficiency was below desirable acceptable commercial levels and the p-doping was particularly undesirable since it reduced the width of the band gap and increased the number of localized states in the band gap.
The prior deposition of amorphous silicon, which has been altered by hydrogen from the silane gas in an attempt to make it more closely resemble crystalline silicon and which has been doped in a manner like that of doping crystalline silicon, has characteristics which in all important respects are inferior to those of doped cyrstalline silicon.
Thus, inadequate doping efficiencies and conductivity were achieved especially in the p-type material, and the photo-voltaic qualities of these silicon films left much to be desired.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been prepared by glow discharge as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, which are incorporated herein by reference, fluorine is introduced into the amorphous silicon semiconductor to substantially reduce the density of localized states therein.
Activated fluorine especially readily diffuses into and bonds to the amorphous silicon in the matrix body, substantially to decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous silicon matrix. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its exceedingly small size, high reactivity, specificity in chemical bonding, and highest electronegativity. Hence, fluorine is qualitatively different from other halogens and so is considered a super-halogen.
As an example, compensation may be achieved with fluorine alone or in combination with hydrogen with the addition of these element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the new alloy so formed has a lower density of defect states in the energy gap than that achieved by the mere neutralizaiton of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the density of defect states which hydrogen contributes while acting as a density of states reducing element. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.